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From Paul Martz, "This change address the following issue: an app opens a Viewer on a multidisplay system, configured to setUpViewAcrossAllDisplays, with a non-default clear mask. In this case, OSG failed to propagate the clear mask to the slave Cameras, resulting in the clear mask being ignored. To fix this issue, this revision adds a new CullSettings::VariablesMask? bit, CLEAR_MASK, to explicitly control inheritance of the clear mask. This bit is set by default, which means that the clear mask now inherits by default, whereas previously it did not."